1. Field of the Invention
The present invention relates to die closing units of injection molding machines, and more particularly to safety covers for such die closing units which, when removed from their closed position, cause the electrical and/or hydraulic controls of the injection molding machine to be blocked, in order to make it impossible to operate the machine, except when the safety cover, or covers, are closed.
2. Description of the Prior Art
It is known to equip die closing units of injection molding machines with safety covers which have a longitudinal control rod attached to the safety cover so as to cooperate with an interlock tumbler which, when not retracted by the action of the cover control rod, prevents the closing of the injection molding die, by blocking the longitudinal movement of a push rod attached to the movable die carrier plate. Such an arrangement if disclosed, for example, in the German Offenlegungsschrift (Published Application) No. 1,529,779, where the push rod of the movable die carrier plate and the control rod of the safety cover thus provide a mechanical interlock action. This prior art device has the shortcoming that it is comparatively easy to render it inoperative, when, in violation of existing safety rules, a machine operator removes or readjusts one or more of the mechanical interlock elements.
More complex safety cover interlock devices feature means for interrupting and inactivating either the hydraulic control circuit, or the electrical controls, or both. However, experience has shown that, inspite of the posting of safety requirements and warnings, and inspite of the known risk of grave personal injury when adjustments are made while the machine is operating, some operators will attempt to inactivate these interlock devices, if the possibility to do so exists. The manufacturer of the machine is powerless to prevent such unauthorized interference.
In U.S. Pat. No. 3,804,003, for example, is disclosed a safety cover interlock device where the die closing unit features two longitudinally movable overlapping safety covers, the removal from the closed position of one of the safety covers causing a control rod to inactivate the hydraulic controls, while a second control member, operatively connected to the safety covers, automatically interrupts the main electrical circuit. This device is so designed that the microswitches which interrupt the main circuit are almost inaccessible to the operating personnel for unauthorized inactivation and tampering. A similar situation exists with respect to the control valve for the hydraulic circuit. No mechanical interlock is provided in this device.